The present invention relates to a method for transmitting messages between mobile stations and a central switching system, and more particularly to a method for transmitting these messages using a more efficient communications link protocol over the air-interface of a cellular telephone system.
In a typical cellular radio system, a geographical area, e.g., a metropolitan area, is divided into several smaller, contiguous radio coverage areas called "cells" . The cells are served by a series of fixed radio stations called "base stations" . The base stations are connected to and controlled by a mobile services switching center (MSC). The MSC, in turn, is connected to the land-line (wire-line) public switched telephone network (PSTN). The telephone users (mobile subscribers) in the cellular radio system are provided with portable (hand-held), transportable (hand-carried) or mobile (car-mounted) telephone units (mobile stations) which communicate voice andlor data with the MSC through a nearby base station. The MSC switches calls between and among wire-line and mobile subscribers, controls signalling to the mobile stations, compiles billing statistics, and provides for the operation, maintenance and testing of the system.
FIG. 1 illustrates the architecture of a conventional cellular radio system built according to the Advanced Mobile Phone Service (AMPS) standard. In FIG. 1, an arbitrary geographic area is divided into a plurality of contiguous radio coverage areas, or cells, CL1-C10. While the system of FIG. 1 is, for illustration purposes, shown to include only ten cells, the number of cells may be much larger in practice. Associated with and located in each of the cells C1-C10 is a base station designated as a corresponding one of a plurality of base stations B1-B10. Each of the base stations B1-B10 includes a plurality of channel units, each comprising a transmitter, a receiver and a controller, as is well known in the art.
In FIG. 1, the base stations B1-B10 are located at the center of the cells C1-C10, respectively, and are equipped with omni-directional antennas transmitting equally in all directions. In this case, all the channel units in each of the base stations B1-B10 are connected to one antenna. However, in other configurations of the cellular radio system, the base stations B1-B10 may be located near the periphery, or otherwise away from the centers of the cells C1-C10 and may illuminate the cells C1-C10 with radio signals directionally. For example, the base station may be equipped with three directional antennas, each one covering a 120-degree sector cell as shown in FIG. 2. In this case, some channel units will be connected to one antenna covering one sector cell, other channel units will be connected to another antenna covering another sector cell, and the remaining channel units will be connected to the remaining antenna covering the remaining sector cell. In FIG. 2, therefore, the base station serves three sector cells. However, it is not always necessary for three sector cells to exist and only one sector cell needs to be used to cover, for example, a road or a highway.
Returning to FIG. 1, each of the base stations B1-B10 is connected by voice and data links to an MSC 20 which is, in turn, connected to a central office (not shown) in the public switching telephone network (PSTN), or a similar facility, e.g., an integrated system digital network (ISDN). The relevant connections and transmission modes between the mobile switching center MSC 20 and the base stations B1-B10, or between the mobile switching center MSC 20 and the PSTN or ISDN, are well known to those of ordinary skill in the art and may include twisted wire pairs, coaxial cables, fiber optic cables or microwave radio channels operating in either analog or digital mode. Further, the voice and data links may either be provided by the operator or leased from a telephone company (telco).
With continuing reference to FIG. 1, a plurality of mobile stations M1-M9 may be found within the cells C1-C10. Again, while only nine mobile stations are shown in FIG. 1, the actual number of mobile stations may be much larger in practice and will generally exceed the number of base stations. Moreover, while none of the mobile stations M1-M9 may be found in some of the cells C1-C10, the presence or absence of the mobile stations M1-M9 in any particular one of the cells C1-C10 depends on the individual desires of each of the mobile subscnbers who may travel from one location in a cell to another or from one cell to an adjacent or neighboring cell.
Each of the mobile stations M1-M9 includes a transmitter, a receiver, a controller and a user interface, e.g., a telephone handset, as is well known in the art. Each of the mobile stations M1-M9 is assigned a mobile identification number (MIN) which, in the United States, is a digital representation of the telephone directory number of the mobile subscriber. The MIN defines the subscription of the mobile subscriber on the radio path and is sent from the mobile station to the MSC 20 at call origination and from MSC 20 to the mobile station at call termination. Each of the mobile stations M1-M9 is also identified by an electronic serial number (ESN) which is a factory-set, "unchangeable" number designed to protect against the unauthorized use of the mobile station. At call origination, for example, the mobile station will send the ESN to the MSC 20. The MSC 20 will compare the received ESN to a "blacklist" of the ESNs of mobile stations which have been reported to be stolen. If a match is found, the stolen mobile station will be denied access.
Each of the cells C1-C10 is allocated a subset of the radio frequency (RF) channels assigned to the entire cellular system by the concerned government authority, e.g., the Federal Communications Commission (FCC) in the United States. Each subset of RF channels is divided into several voice or speech channels which are used to carry voice conversations, and at least one paging/access or control channel which is used to carry supervisory data messages, between each of the base stations B1-B10 and the mobile stations M1-M9 in its coverage area. Each RF channel comprises a duplex channel (bi-directional radio transmission path) between the base station and the mobile station. The RF channel consists of a pair of separate frequencies, one for transmission by the base station (reception by the mobile station) and one for transmission by the mobile station (reception by the base station). Each channel unit in the base stations B1-B10 normally operates on a preselected one of the radio channels allocated to the corresponding cell, i.e., the transmitter (TX) and receiver (RX) of the channel unit are tuned to a pair of transmit and receive frequencies, respectively, which is not changed. The transceiver (TXIRX) of each mobile station M1-M9, however, may tune to any of the radio channels specified in the system.
In typical land-line systems, remote stations and control centers are connected by copper or fiber optic circuits which have a data throughput capacity and performance integrity that is generally significantly better than the data throughput capacity and performance integrity provided by an air interface in a cellular telephone system. As a result, the conciseness of overhead required to manage any selected communication link protocol for land-line systems is of secondary importance. In cellular telephone systems, an air interface communications link protocol is required in order to allow a mobile station to communicate with a cellular switching system. A communications link protocol is used to initiate and to receive cellular telephone calls.
The electromagnetic spectrum available for use by cellular telephone systems is limited and is portioned into units called channels. Individual channels are used as communication links either on a shared basis or on a dedicated or reserved basis. When individual channels are used as communication links on a shared basis, multiple mobile stations may either listen to or contend for the same channels. In the contending situation, each shared channel can be used by a plurality of mobile stations which compete to obtain exclusive use of the channel for a limited period of time. On the other hand, when individual channels are used as communication links on a dedicated basis, a single mobile station is assigned the exclusive use of the channel for as long as it needs it.
The continued need to serve existing analog-only mobile stations has led to the specification in IS-54B of an analog control channel (ACC) which has been inherited from the prior AMPS or the equivalent EIA/TIA-553 standard. According to EIA/TIA-553, the analog forward control channel (FOCC) on the down-link from the base station to the mobile stations carries a continuous data stream of messages (words) in the format shown in FIG. 3. Several different types (functional classes) of messages may be transmitted on the analog FOCC. These messages include a system parameter overhead message (SPOM), a global action overhead message (GAOM), a registration identification message (REGID), a mobile station control message, e.g., a paging message, and a control-filler message. The SPOM, GOAM and REGID are overhead messages which are intended for use by all mobile stations in the coverage area of the base station. Overhead messages are sent in a group called an overhead message train (OMT). The first message of each OMT must always be the SPOM which is transmitted every 0.8.+-.0.3 seconds.
The format of the analog FOCC shown in FIG. 3 requires an idle mobile station listening to the FOCC to read all the messages transmitted in each OMT (not just paging messages) even though the information contained in these messages may not have changed from one OMT to the next OMT. This requirement tends to unnecessarily limit the mobile station battery life. One of the goals of the next generation digital cellular systems is to extend the "talk time" for the user, that is, the battery life of the mobile station. To this end, U.S. patent application Ser. No. 07/956,640 (which is incorporated here by reference) discloses a digital FOCC which can carry the types of messages specified for the analog FOCC, but in a format which allows an idle mobile station to read overhead messages when locking onto the FOCC and thereafter only when the information has changed, and to enter "sleep mode" at all other times. While in sleep mode, the mobile station turns off most internal circuitry and saves battery power.
The above-referenced U.S. patent application Ser. No. 07/956,640 shows how a digital control channel (DCC) may be defined alongside the digital traffic channels (DTC) specified in IS-54B. Referring to FIG. 4, a half-rate DCC would occupy one slot, while a full-rate DCC would occupy two slots, out of the six slots in each time-division-multiple-access (TDMA) frame of duration 40 milliseconds (msec). For additional DCC capacity, additional half-rate or full-rate DCCs may be defined in place of the DTCs until there are no more available slots on the carrier (DCCs may then be defmed on another carrier if needed). Each IS-54B RF channel, therefore, can carry DTCs only, DCCs only, or a mixture of both DTCs and DCCs. Within the IS-54B framework, each RF channel can have up to three full-rate DTCs/DCCs, or six half-rate DTCs/DCCs, or any combination in-between, for example, one full-rate and four half-rate DTCs/DCCs.
In general, however, the transmission rate of the DCC need not coincide with the half-rate and full-rate specified in IS-54B, and the length of the DCC slots may not be uniform and may not coincide with the length of the DTC slots. FIG. 5 shows a general example of a forward (or downlink) DCC configured as a succession of time slots 1, 2, . . . , N, . . . included in the consecutive time slots 1, 2, . . . sent on a carrier frequency. These DCC slots may be defined on a radio channel such as that specified by IS-54B, and may consist, as seen in FIG. 5 for example, of every n-th slot in a series of consecutive slots. Each DCC slot has a duration that may or may not be 6.67 msec, which is the length of a DTC slot according to the IS-54B standard. (There are six DTC slots in each 40-msec TDMA frame.) Alternatively (and without limitation on other possible alternatives), these DCC slots may be defined in other ways known to one skilled in the art.
As shown in FIG. 5, the DCC slots may be organized into superframes and each superframe may include a number of logical channels that carry different kinds of information. One or more DCC slots may be allocated to each logical channel in the superframe. The exemplary downlink superframe in FIG. 5 includes three logical channels: a broadcast control channel (BCCH) including six successive slots for overhead messages; a paging channel (PCH) including one slot for paging messages; and an access response channel (ARCH) including one slot for channel assignment and other messages. The remaining time slots in the exemplary superframe of FIG. 5 may be dedicated to other logical channels, such as additional paging channels PCH or other channels. Since the number of mobile stations is usually much greater than the number of slots in the superframe, each paging slot is used for paging several mobile stations that share some unique characteristic, e.g., the last digit of the MIN.
For purposes of efficient sleep mode operation and fast cell selection, the BCCH may be divided into a number of sub-channels. U.S. patent application Ser. No. 07/956,640 discloses a BCCH structure that allows the mobile station to read a minimum amount of information when it is switched on (when it locks onto a DCC) before being able to access the system (place or receive a call). After being switched on, an idle mobile station needs to regularly monitor only its assigned PCH slots (usually one in each superframe); the mobile can sleep during other slots. The ratio of the mobile's time spent reading paging messages and its time spent asleep is controllable and represents a tradeoff between call-set-up delay and power consumption.
Since each TDMA time slot has a certain fixed information carrying capacity, each burst typically carries only a portion of a layer 3 message as noted above. In the uplink direction, multiple mobile stations attempt to communicate with the system on a contention basis, while multiple mobile stations listen for layer 3 messages sent from the system in the downlink direction. In known systems, any given layer 3 message must be carried using as many TDMA channel bursts as required to send the entire layer 3 message.
The communication link protocol is commonly referred to as a layer 2 protocol within the communications industry and its functionality includes the limiting or framing of higher level messages. Traditional layer 2 protocol framing mechanisms or bit stuffing in flag characters are commonly used in land- line networks today to frame higher layer messages, which are referred to as layer 3 messages. These layer 3 messages may be sent between communicating layer 3 peer entities residing within mobile stations and cellular switching systems.
For a better understanding of the structure and operation of the present invention, the digital control channel DCC may be divided into three layers: layer 1 (physical layer), layer 2, and layer 3. The physical layer (L1) defmes the parameters of the physical communications channel, e.g., RF spacing, modulation characteristics, etc. Layer 2 (L2) defines the techniques necessary for the accurate transmission of information within the constraints of the physical channel, e.g., error correction and detection, etc. Layer 3 (L3) defines the procedures for reception and processing of information transmitted over the physical channel.
FIG. 6 schematically illustrates pluralities of layer 3 messages 11, layer 2 frames 13, and layer 1 channel bursts, or time slots, 15. In FIG. 6, each group of channel bursts corresponding to each layer 3 message may constitute a logical channel, and as described above, the channel bursts for a given layer 3 message would usually not be consecutive slots on an IS-54B carrier. On the other hand, the channel bursts could be consecutive; as soon as one time slot ends, the next time slot could begin.
Each layer 1 channel burst 15 contains a complete layer 2 frame as well as other information such as, for example, error correction information and other overhead information used for layer 1 operation. Each layer 2 frame contains at least a portion of a layer 3 message as well as overhead information used for layer 2 operation. Although not indicated in FIG. 6, each layer 3 message would include various information elements that can be considered the payload of the message, a header portion for identifying the respective message's type, and possibly padding.
Each layer 1 burst and each layer 2 frame is divided into a plurality of different fields. In particular, a limited-length DATA field in each layer 2 frame contains the layer 3 message 11. Since layer 3 messages have variable lengths depending upon the amount of information contained in the layer 3 message, a plurality of layer 2 frames may be needed for transmission of a single layer 3 message. As a result, a plurality of layer 1 channel bursts may also be needed to transmit the entire layer 3 message as there is a one-to-one correspondence between channel bursts and layer 2 frames.
As noted above, when more than one channel burst is required to send a layer 3 message, the several bursts are not usually consecutive bursts on the radio channel. Moreover, the several bursts are not even usually successive bursts devoted to the particular logical channel used for carrying the layer 3 message.
In light of the generally reduced data throughput capacity and performance integrity afforded by an individual channel in a channel sharing situation in a cellular telephone environment, the selection of an efficient air interface protocol to serve as the basis of the communication link becomes paramount.
Thus, there is a need for a layer 2 header which describes what is contained in the time slot, how it is contained in the time slot and how the information should be interpreted.